@Julianmignone78 No matter the place you bought it :)
if you are a beginner in the modding world, I think it will be better for you to read some tutorials :) There is plenty of them on the web, learn basics :
- OpenIV software
- Necessary files to make mods functionnable
...
If you want some help, you can contact me on Facebook (to not polluate the thread), link on my profile

@Julianmignone78 It depends the way you install the cars : if you install with the Add-On way, you will be the only one to drive the car, if you choose the Replace way, you will see the car in the traffic. All cars have not replace versions, and vice versa.
Otherwise, you can find mod that make Add-on car spawning in the traffic, but I did not test them.

@LorenVidican Ok It was my fault x) I did not install the script file you mentionned to download. You should mentionned it as a step in the instructions file, to be more understandable :) otherwise, like me, we skip the last sentence ! ^^
5 stars !